1. Corneal endothelial cell membrane electrical resistance is estimated at about 400 M omega by using intracellular micro-electrodes. 2. If all ion diffusion across the endothelium were transcellular, electrical resistance of the endothelial monolayer would be about 1300 omega cm2, as there are about 3000 cells mm-2. 3. Measured endothelial monolayer resistance is 12.7 +/- 0.8 omega cm2 (mean +/- S.E., n = 6), which indicates that about 99% of ion diffusion does not cross the transcellular pathway, but must pass through the paracellular route. Arguments are presented which suggest that the proportion may be even higher. 4. Endothelial passive ion permeabilities are about the same as corneal stromal passive ion permeabilities. 5. Stromal water diffusional permeability is about the same as stromal ion permeability, after allowance is made for free diffusion coefficients. In contrast, endothelial water diffusional permeability is so high as to be unmeasurable. 6. It is concluded that ions diffuse across the corneal endothelium through the paracellular route, and that water diffuses across the endothelium mainly through the transcellular route.
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